The toilet flange backset is not specified by code but is determined by the physical requirements of the toilet. For most toilets the correct spacing is 12" from the finished wall, however, in instance where you cannot get the 12" backset they make toilets with a 10" setback.
Offset flanges are prohibited.
The toilet flange should be set with the underside of the flange flush with the finished floor, which leaves the top 1/4 to 3/8" above the finished floor. The code prohibits the flange top from being flush with or recessed below the finished floor.
There are a couple other dimensions you should be aware of. The codes require a minimum of 15" from the center of the closet flange to each side.
Depending upon what codes are in force in your locality there is a frontal clearance that is measured from the front edge of the bowl to the finished wall.
The International Residential Code frontal clearance is 21"
The Uniform Plumbing Cocde frontal clearance is 24"
Before you close the wall, there may also be a problem with that 2" vent. Can you give more details on what it is connected to and how it runs and height above the floor.
